Discount Calculator
Enter the original price and the discount to see what you’ll pay and what you save — or enter two prices to find the percentage off. Stacked discounts are handled correctly, and you can work backwards from a sale price to the original, which is where mental arithmetic most often goes wrong.
How it works
Sale price = price × (100 − discount) ÷ 100. A £120 jacket at 30% off is 120 × 0.70 = £84, saving you £36.
Percentage off = (original − sale) ÷ original × 100. A drop from £250 to £187.50 is (62.50 ÷ 250) × 100 = 25% off.
Stacked discounts multiply rather than add: 20% off plus an extra 10% at checkout is 28% off in total, not 30% (0.80 × 0.90 = 0.72). The calculator shows the true combined percentage.
Working backwards needs division, not addition: original price = sale price ÷ (1 − discount). If a jacket costs £84 after 30% off, the original was 84 ÷ 0.70 = £120. The common mistake is adding 30% back onto £84, which gives £109.20 — wrong, because the 30% was taken from the larger original figure, not from the sale price.
Fraction discounts follow the same rule: a third off £90 is 90 × 2/3 = £60, and “half price plus an extra 10%” retains 0.50 × 0.90 = 0.45 of the price — a 55% total saving, not 60%. For quick mental checks, find 10% by shifting the decimal point one place and scale from there: 5% is half of that, 20% is double, 25% is a quarter of the price.
For any inputs the calculator shows the final price, the cash saving and the true percentage off — including the combined figure for stacked promotions. Treat headline percentages sceptically when the “was” price looks inflated: UK pricing-practice guidance (CMA/CTSI) says a was-price should be one the item genuinely sold at recently, so the honest measure of a deal is the discount from the price actually charged, not from a label.
FAQ
How do I work out 20% off a price?
Multiply the price by 0.80: a £65 item at 20% off costs £52. The general rule is price × (100 − discount) ÷ 100. A quick mental shortcut is to find 10% by moving the decimal point one place (£6.50), double it (£13) and subtract that saving from the original price.
Why is 50% off plus 20% off not 70% off?
Because the second discount applies to the already-reduced price. A £100 item at 50% off costs £50; a further 20% off that is £40 — a total saving of 60%, not 70%. Retailers rely on this difference in “extra % off sale items” promotions, so always check the final price rather than adding the percentages.
How do I check a sale price is genuine?
Compare against the price history, not just the ticket. UK guidance (CMA/CTSI pricing practices) says a “was” price should be the price the item genuinely sold at recently. Work out the real percentage off with this calculator — if a “70% off” claim is measured against an inflated was-price, the true discount from the typical selling price is often far smaller.
How do I find the original price before the discount?
Divide the sale price by (1 − the discount as a decimal): £52 after 20% off means 52 ÷ 0.80 = £65 originally. Do not add the percentage back on — 20% added to £52 gives only £62.40, not £65, because a percentage of a smaller base is a smaller amount of money. The general rule is original = sale ÷ (1 − discount).
What is 25% off £80?
£60 — the saving is 80 × 0.25 = £20 and the sale price is 80 × 0.75. A tidy mental route for 25% is to halve the price and halve it again (£80 → £40 → £20 of saving). The same pattern covers any quarter-based discount: 75% off leaves exactly a quarter of the price, £20 in this case.
Which is better: £15 off or 15% off?
It depends on the price — the two are equal at exactly £100, where 15% is £15. Below £100 the flat £15 saves more (15% of £60 is only £9); above £100 the percentage wins (15% of £200 is £30). Convert both offers into cash saved on the actual price you are paying and compare like with like.